Remove executor, add buy/sell to Alpaca, move capital allocation to Bot

Replace the Executor logging placeholder with real buy/sell methods on
the Alpaca class that place market orders via createOrder and return the
fill price. Strategies now receive their capital amount directly and
place orders themselves. Bot accepts StrategyAllocation[] to decouple
capital allocation from strategy definition.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/trading.test.ts

@@ -12,6 +12,8 @@ function mockAlpaca(overrides: Partial<Alpaca> = {}): Alpaca {
         getLatestBid: vi.fn(),
         getLatestSpread: vi.fn(),
         getLatestTrades: vi.fn(),
+        buy: vi.fn(),
+        sell: vi.fn(),
         ...overrides,
     } as unknown as Alpaca;
 }